The present invention relates to a moulded coil and a production method thereof.
One example of this type of moulded coil is described in JP 5-11410. As shown in FIG. 10 of this specification a coil 5 thereof is formed by winding a copper coil wire 4 around a plastic bobbin 1 comprising a cylindrical body 2 which has flanges 3 positioned near both ends. A sheathing 6 is moulded around the coil 5 in the following way. A mould presses upper and lower ends of the cylindrical body 2 of the bobbin 1, thereby maintaining the coil 5 within a cavity of the mould. The cavity is filled with molten plastic, which hardens to form the sheathing 6.
The components moulded in the conventional manner have a joint 7 between the bobbin 1 and the sheathing 6. If the materials that constitute the bobbin 1 and the sheathing 6 do not match, there is the danger that the bobbin 1 and the sheathing 6 do not fit adequately and are not fitted tightly together, this causing a gap to form in the joint 7. In such a case, if, for example, the moulded coil is used in a manner whereby it is immersed in oil, this oil may enter through the gap, thereby damaging the coil wire 4, particularly if the oil contains sulphur.
The present invention has taken the above problem into consideration, and aims to improve on the conventional example.
According to the invention there is provided a method of encapsulating an electrical coil comprising a bobbin having a coil wire wound thereon, and from which electrical connections extend, the method comprising the steps of:
placing the coil in a mould cavity;
spacing the coil away from the wall of the cavity by means of movable supports, said supports occupying a predetermined volume of said cavity;
introducing molten plastic into the cavity;
moving said supports away from the coil after a predetermined percentage volume of said cavity is filled; and
continuing injection of molten plastic so that said predetermined volume is filled and said coil is encapsulated, the electrical connections protruding from the coil encapsulation.
Such a method encapsulates the coil and prevents leakage through joints or junction faces.
Preferably the necessary electrical connections from the coil are encapsulated in a tongue to further reduce the possibility of leakage. Such a tongue increases the length of any possible leakage path.
The invention also provides an encapsulated electrical coil produced by the method of the invention.
In a preferred embodiment this coil includes a tongue to encapsulate the necessary electrical connections, and this tongue may define a plug adapted to receive a sheath for wires which are in use connected to said connections. The plug and sheath arrangement further improve resistance to leakage.